Ultra-fine ceramic particles, with particle sizes of less than about 300 nanometers, are useful for many different purposes. Thus, for example, as is disclosed in U.S. Pat. No. 5,061,682 of Aksay et al., "The use of high temperature superconducting ceramic articles and nonsuperconducting ceramic articles requires reproducible production of the articles from ceramic powders with high densities, high purities, good homogeneity, and fine grain size" (see column 1 of this patent).
However, the prior art methods for producing ultra-fine ceramic particles are not entirely satisfactory. Thus, as is disclosed in column 1 of this patent, "A common method for producing superconductive ceramic powders involves grinding ceramic powders . . . . The grinding produces particles that are greater than one micron in size, are not equiaxed, have a broad particle size distribution, and are often contaminated by the grinding media. Techniques such as sol-gel, precipitation, and freeze-drying have been developed to overcome some of these undesirable features . . . ; however, most of these alternative techniques cannot directly produce superconductive ceramic particles."
The Aksay et al. patent, which is based upon an application filed in 1990, attempts to solve the problems of the prior art by a process which involves the steps of forming droplets of a ceramic precursor mixture, removing solvent from such mixture to form particles, and thereafter thermally initiating an anionic oxidation-reduction reaction. The ceramic precursor mixture used in this process contains a metal cation, a carbohydrate, a solvent, and an anion capable of participating in an anionic oxidation-reduction reaction with the carbohydrate.
However, the particles produced by the process of the Aksay et al. patent have a specific surface area (as measured by the BET nitrogen-adsorption test) which is relatively low. Furthermore, with this process, it is difficult to completely remove all of the solvent from the ceramic precursor material. This fact often causes processing problems such as, e.g., the deposition of the reactor with precursor material, contamination of the precursor material, etc.
It is an object of this invention to provide a process for producing ultra-fine ceramic particles which are substantially finer than the particles produced by the process of Aksay et al.
It is another object of this invention to provide a process for producing ultra-fine ceramic particles in which the precursor material is substantially dry and, consequently, more reactive.
It is another object of this invention to provide a process for producing ultra-fine ceramic particles in which the thermal initiation step is physically removed from the solvent removal step and the risk of unintended explosion is thereby minimized.
It is another object of this invention to provide a process for producing ultra-fine ceramic particles in which, during the solvent removal step, the deposition of precursor material upon the walls of the dryer is minimized.
It is another object of this invention to provide a process for producing coated, ultra-fine ceramic particles.
It is another object of this invention to provide a process for firing the ultra-fine ceramic particles so that, during such firing, the grain size of the ceramic particles does not substantially increase.